


Be Careful What You Wish For, Sam Winchester

by Lopsided_Whiskey_Grin



Category: Supernatural
Genre: Anal Fingering, Anal Sex, Angst, Bottom Castiel, Cheating, First Time Bottoming, Infidelity, M/M, Sex Pollen, Top Sam Winchester, Unrequited Love
Language: English
Status: Completed
Published: 2016-09-29
Updated: 2016-09-29
Packaged: 2018-08-18 11:28:51
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,619
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8160550
Author URL: https://archiveofourown.org/users/Lopsided_Whiskey_Grin/pseuds/Lopsided_Whiskey_Grin
Summary: Sam has had feelings for Cas since the day they met even though he knows he can never act on them; he would never do anything to hurt Dean. One fateful day though when he and Cas are doing research in the Men of Letters library, Cas stumbles upon an ancient relic that will change everything. 
Fulfilled for a tumblr prompt: a Sastiel one shot of where Castiel is caught in the act of cheating on Dean with his own brother, Sam. Don't be too light on the Sastiel sex scene though; I like to read about how they got into the mess and what they're doing between the sheets ;) Thank you, and keep up the fantastic work!! All the love. Xx





	

It was quiet in the library where Sam and Cas were looking through old manuscripts. They were doing research on a monster neither of them had heard of and relaying all the information they could find to Dean, who was two towns over, through phone calls and texts. The creature was a stubborn sonofabitch and it took Sam and Cas hours and hours of pouring through books and scrolls to try to find the exact weapon needed for Dean to finally kill the damn thing. 

Sam didn’t mind the tediousness of the task one bit. He loved the brain-work of leafing through all the old papers and learning more about the Men of Letters. He also loved watching Cas as he scanned through the documents. He found his gaze drifting over to him more than once as they sat across from each other at the long table. He was captivated by the deep blue eyes darting over hundred-year old letters, the soft pink bottom lip catching between white teeth, the displacement of dark locks by restless fingers. 

He'd be lying if he said he hadn't always felt  _ something  _ for Cas. Hell, the very first moment they had met and Sam settled his hand in Cas’ he had felt an inexplicable tightness in his chest that he could only describe as longing. But it was painfully obvious that Cas and Dean belonged together. In all honesty, it had been pretty amazing to see the relationship between his brother and the angel blossom the way it had, and Sam would be damned if he ever came between them. He would  _ never _ hurt Dean in such a way, no matter how much his heart and body ached for just one kiss from Castiel. And it wasn’t like Cas even held any romantic interest in Sam anyway. It just wasn’t in the cards.

Sam sighed, set aside the ancient book he had been looking through, and picked up the next one in the pile. He would just have to settle with admiring Cas from afar. Surely it wouldn't kill him, right? And he was pretty certain there wasn't anything wrong with fantasizing about Cas at the end of the day when he lay alone in his bed and could hear the soft gasps and moans drifting down the hall from Dean's room. As long as he kept all those dirty thoughts in his head, he wasn't hurting anyone, right? 

Another hour passed, with Sam still shooting furtive glances Cas’ way, and they had almost cracked the mystery of the weapon needed to kill the monster. Cas found a yellowed piece of paper, dry and brittle as an autumn leaf, that looked promising but it was ripped in half with the bottom missing. Sam flew through the rest of the volumes on the shelf nearest him and found the matching piece tucked in the pages of one of the last books. 

He let out a relieved sigh as he handed it to Cas and felt his chest tighten when a broad and beautiful smile broke across Cas’ face when he realized they solved the mystery. Sam had to pull up all the self-control he could muster not to kiss Cas right then and there. He looked so irresistible with that delighted glint in his eye and a delectable pink flushing his cheeks. 

Cas excitedly grabbed up the cell phone and dialed Dean. Sam couldn't look away from the pleased expression suffusing Cas’ face as he gave Dean all the details in his low rumbling voice. The deep timbre of it sent tingles all up and down Sam's spine as he listened. He imagined the way Cas’ voice would sound moaning his name. 

By the time the call was over Sam felt a little breathless and more than a little hard. Blinking to bring himself back to reality, he looked up at Cas as soon as he hung up the phone. 

“Dean got it covered?” he asked after clearing his throat.

Cas smiled fondly at the cell in his hand and Sam's heart ached with wanting Cas to look at him that way. Cas nodded then and brought his gaze up to Sam's. “He has it taken care of, thanks to us. He'll be heading back soon.”

Sam quickly looked away, needing to busy himself with something before he got lost in Cas’ eyes. “Want to help me clean up these books? Maybe we can persuade Dean to get us some dinner on his way and we'll have a place cleared off where we can eat.”

“Of course,” Cas replied with a smile that warmed Sam deep down inside. 

They moved around the library with a quiet ease, reshelving books and scrolls back on the dusty bookcases. Suddenly there was a crash across the room and Sam jerked his head up to see that Cas had bumped into one of the shelves and knocked a stack of books to the floor along with a handful of ancient relics. Sam hurried over to help him, kneeling down beside him to start picking things up. 

“Sam, what's this?” Cas asked, going still when he picked up a small clear vial from the floor. It appeared to contain browned and brittle flower petals half a breath from disintegrating. 

Sam shrugged his shoulders. “No clue. Probably shouldn't open it thou..”

Before Sam had even finished his sentence, Cas was pulling the cork from the vial, his curiosity apparently getting the better of him. A sweetly fragrant cloud floated from the opened bottle and Sam instantly felt lightheaded. He pushed to his feet just as Cas did, his vision fuzzy for a moment before it cleared. 

And then Cas was advancing on him, pushing his hands up into Sam's hair as soon as their bodies were touching, pulling Sam's head down for a kiss. Confusion and elation warred in Sam's mind for a split second. It was everything he had ever wanted -- just to feel Cas’ lips moving against his own. But lucid thought won out. This was not right, not at all. What the hell was going on here? 

Sam put his palms flat on Cas’ shoulders and pushed him back. It took more strength than Sam thought it would. “Cas! What are you doing?” Even as he said it though, he wanted to take it back. He wanted Cas’ lips on his again. He wanted more, he wanted it all. 

Cas’ gasped in a breath and looked up at him through his dark lashes. “I do not know, Sam. I only know it feels like I'm going to die if I can't touch you right now.”

Sam felt the same way. His body _ screamed _ for Cas. Giving his head a rough shake to desperately try to clear his thoughts, he took a step back. It must have been the flower petals, it was the only thing that made sense, because no matter how badly he wanted Cas he knew in his heart he couldn't have him. Scrubbing his hands down his face, he turned away. 

“We need to go outside, get some fresh air,” he said with an unsteady voice that lacked much conviction. 

But he was suddenly being tugged back gently by Cas’ hand curling around his. He spun around slowly until he was facing Cas again. And it was so fucking  _ easy _ this time to just let his body fall against Cas, to melt into him, to bring his hands up to Cas’ face and coax his mouth open and take and take and  _ take  _ until they were both breathless. 

He pulled back to gasp for air and rested his forehead on Cas’, feeling the angel’s fingers digging into the back of his shirt. 

“We can't do this,” Cas panted, even as he rucked Sam's shirt up and slid his hands down the back of his jeans. 

Sam sucked in a sharp breath at the feel of Cas’ hands on his overheated skin and dropped his head down to lave his tongue along the side of Cas’ neck. Cas moaned and tilted his chin at an angle to give Sam better access to his throat. 

“I - I can't stop, Cas,” he stuttered between bites to the sensitive skin at Cas’ pulse point. And he really, honest to God, could not fucking  _ stop.  _ The effects of the flower were sending wave after wave of desire cresting through him, and while he might have had the mental fortitude to put a stop to it at first, he felt his control slipping farther away with each passing second. He couldn't really remember anymore why they shouldn't be doing this, especially when it felt so damn good. 

Grinding his hips forward, Sam growled low in his chest when he felt the hardness of Cas’ erection pressing against his own. They were wearing far too many fucking clothes. 

“Pants off,” he rasped. Pure, unadulterated need pounded through him as he jerked his hands down to unclasp Cas’ belt and undo his trousers. He didn't have the patience for a soft, slow lovemaking session; he was barely sure he'd be able to get Cas’ ass naked enough to bury himself inside in enough time before he died from the arousal scorching through him. 

Cas made a noise of approval and went to work on Sam's jeans. Soon they were shoved down to his thighs along with his boxers and Sam positively could not wait any longer. He turned Cas around roughly and bent him over the library table. 

Cas grunted and swayed his hips. “Sam,  _ please.” _

Sam released a shaky, pent up breath, running his palms down Cas’ cheeks. They were pliable under his touch, the skin creamy and smooth. He pulled them apart to expose Cas’ hole and felt Cas shudder against his hands. The tightly furled flesh twitched and Sam's mouth instantly flooded with saliva. 

“I've never been on the ‘receiving’ end,” Cas rasped suddenly, turning his head to look at Sam over his shoulder. 

Sam's knees went weak. “Really?” he asked in wonder. “With Dean you're…?” His question trailed away as his lust-addled brain cleared for a moment and guilt and shame filled his mind with images of his brother and what he was about to do with Cas. 

But just as quickly Dean was forgotten when Cas pushed his ass back toward Sam enticingly. All thought and reason was wiped from Sam's memory and was instantly replaced by need and overwhelming desire. Bending forward, he hawked a thick wad of spit onto Cas’ hole and rubbed it in circles, dipping just the very tip of his forefinger inside. A deep, possessive stripe of male satisfaction plowed through him at the thought of his cock being the first inside of Cas.

Cas bucked beneath him and let out a heavy moan. “Please just hurry, Sam,” he pleaded. “I can't wait much longer. I need to feel you'.”

The very last vestige of Sam's control snapped and he was reduced to pure and unapologetic instinct. He pumped his finger back inside, adding a second soon after, prepping Cas as quickly as possible. Cas cried out beneath him, reaching a hand under himself. Sam could see the bunch of muscles in his arm as Cas began stroking his cock and it only drove Sam higher into his desperation. Taking his own dick in hand, he pulled his fingers free, watching hungrily as Cas’ widened hole try to close around nothing. 

Pressing the swollen head of his cock against Cas’ entrance, Sam said, “Take a deep breath and try to relax.”

He watched Cas nod then breached him with one long thrust. They were much too ravenous to wait any longer. Twin groans echoed off the walls in the library when Sam bottomed out on that first punishing stroke. 

“Oh god, Sam,” Cas keened, his voice straining, “I feel so… _ full _ .”

Sam latched onto Cas’ hips, gritting his teeth together from the almost overwhelming pleasure of feeling Cas’ warm, wet insides clench around him as Cas tested the size of the intrusion. “Fuck,” he breathed raggedly, pulling out halfway then slamming back in. 

Cas arched his back, pulling Sam in impossibly deeper and they both gasped. Sam moved his hands from Cas’ hips up to his shoulders, anchoring himself firmly against Cas and using his long arms to hold Cas steady as he began pounding in and out. Cas continued to stroke himself with one hand while the other formed a fist and slammed loudly down onto the table. Sam knew then that he had hit Cas’ prostate. 

An incomprehensible string of words poured from Cas’ lips as Sam continued drilling his ass and Sam wondered if he was speaking english anymore or if he had slipped into enochian; whatever it was, it sounded hot as fucking hell.  Keeping his grip tight on Cas’ shoulders, Sam angled his hips to try and hit Cas’ sweet spot on every other plunge forward to hear more of those sweet cries. 

All too soon, and somehow also not soon enough, Sam felt his guts tighten and his balls draw up close to his body. His hands fell back to Cas’ hips, holding on tight and bouncing Cas up and down on his cock as fast as he could. Cas came before him, his whole body clenching with a long low groan that reverberated straight through Sam’s chest. Sam tipped over the edge then too, slamming in as deep as he could go, pulsing out his release into Cas’ throbbing warmth with a shuddering growl. The tight ring of muscle around his dick constricted in time with Cas’ panting breaths, milking Sam of every single last drop.  

But not more than a second after he emptied himself into Cas than did his head clear from the hold the ancient flower petals had on him. He looked down in confusion and horror to see himself buried to the hilt in brother’s lover’s ass. Cas seemed to be gaining his senses as well because he immediately stilled, looking back over his shoulder with wide, ashamed eyes. 

“Holy shit, Cas, what did we just do?” Sam choked. He started gingerly pulling his softening cock from Cas but there was a sudden loud crash behind him. 

Jerking his head to the side, he saw Dean in the entryway, a smashed sixpack lying on the ground at his feet. Beer was splashed and foaming all around Dean’s boots but he was paying it no mind at all. He was laser focused on the sight before him and rage and heartbreak poured off of him in waves. 

Sam’s heart plummeted and he instantly felt nauseated. His mind had been so far gone he had almost thought it was all a dream, that maybe he had drifted off to sleep while he was helping Cas with the research and his mind was playing out a very vivid fantasy. But it was all real -- the pain and betrayal in Dean’s eyes was real, the shaking of Cas under his hands as he began sobbing where he was still bent over the table was real, the sinking hopeless feeling wrenching through Sam’s chest was very fucking real. It was all like a nightmare he couldn’t escape. 

Sam stepped back from Cas, pulling his jeans up as fast as he could with his trembling and uncoordinated hands. “Dean, I can explain. There were these flower petals…”

Dean did not let him finish. His hands were fisted dangerously at his sides and a tick bunched at the back of his jaw. “You have five minutes to get your shit and get the fuck out.”

They were the last words Dean spoke to him for almost six months.    
  



End file.
